Control of sigma factor activity during Bacillus subtilis sporulation

When starved, Bacillus subtilis undergoes asymmetric division to produce tw o cell types with different fates. The larger mother cell engulfs the small er forespore, then nurtures it and, eventually, lyses to release a dormant, environmentally resistant spore. Driving these changes is a programme of t ranscriptional gene regulation. At the heart of the programme are a factors , which become active at different times, some only in one cell type or the other, and each directing RNA polymerase to transcribe a different set of genes. The activity of each a factor in the cascade is carefully regulated by multiple mechanisms. In some cases, novel proteins control both a factor activity and morphogenesis, co-ordinating the programme of gene expression with morphological change. These bifunctional proteins, as well as other p roteins involved in a factor activation, and even precursors of a factors t hemselves, are targeted to critical locations, allowing the mother cell and forespore to communicate with each other and to co-ordinate their programm es of gene expression. This signalling can result in proteolytic a factor a ctivation. Other mechanisms, such as an anti-a factor and, perhaps, proteol ytic degradation, prevent a factors from becoming active in the wrong cell type. Accessory transcription factors modulate RNA polymerase activity at s pecific promoters. Negative feedback loops limit a factor production and fa cilitate the transition from one a factor to the next. Together, the mechan isms controlling a factor activity ensure that genes are expressed at the p roper time and level in each cell type.